Gay marriage defeats the purpose of marriage. Marriages were built upon the idea of heterosexual couples uniting to raise a family. People who are pro gay marriage usually have the claims that marriage is about love, but this is far from true. If marriage is about love, then why is the state involved? They are involved due to some sort of interest. If the goverment cared about love, then they would attempt to regulate and control friendships or non marital relationships. The reason they regulate marriage over these other relationships is because marriage is where you are meant to produce a larger workforce.
People should have the right to marry who they want to. Gay couples share the same love as heterosexual couples. Nothing should be able to put a hold on their ability to get married. Many gay couples adopt children to raise as their own. Many kids are put up for adoption and since gay couples can't reproduce they're own, they adopt children who don't have caregivers of their own. This is advantageous because these kids are put into homes with people who will love and care for them. I have heard many stories of kids who have same-sex parents. One wouldn't even be able to tell that they had parents of the same sex; they aren't raised any different.
If the couple lives together, maintain a household and raise children just as a heterosexual couple does, why can't they get married just like this couple. The only difference in this situation is solely that one person is not the opposite sex. Considering that there is not much to differentiate of the two couples, why treat them differently and give more rights and benefits to only one. The outcome of both marriages would have a minimal difference. They are both in it for the same reasons.